Abstract

Background

When females mate with multiple partners, sperm from rival males compete to fertilise
the ova. Studies of experimental evolution have proven the selective action of sperm
competition on male reproductive traits. However, while reproductive traits may evolve
in response to sperm competition, this does not necessarily provide evidence that
sperm competitive ability responds to selection. Indeed, a study of Drosophila failed to observe divergence in sperm competitive ability of males in lines selected
for enhanced sperm offence and defence.

Results

Adopting the naturally polygamous house mouse (Mus domesticus) as our vertebrate model, we performed an experimental evolution study and observed
genetic divergence in sperm quality; males from the polygamous selection lines produced
ejaculates with increased sperm numbers and greater sperm motility compared to males
from the monogamous lines. Here, after 12 generations of experimental evolution, we
conducted competitive matings between males from lineages evolving under sperm competition
and males from lineages subject to relaxed selection. We reduced variation in paternity
arising from embryo mortality by genotyping embryos in utero at 14 days gestation. Our microsatellite data revealed a significant paternity bias
toward males that evolved under the selective regime of sperm competition.

Conclusion

We provide evidence that the sperm competitiveness phenotype can respond to selection,
and show that improved sperm quality translates to greater competitive fertilisation
success in house mice.